This invention relates to an encoder of a multi-pulse type for use in encoding a speech signal into a plurality of excitation pulses specifying an exciting source or a vocal tract through which the speech signal is produced.
A conventional encoder of the type described is disclosed in U.S. application Ser. No. 726,583 filed Apr. 23, 1985, by Tanaka et al and assigned to the instant assignee. The encoder is used in general for a data transmission system in combination with a decoder which is used as a counterpart of the encoder.
In the encoder, the speech signal is divided into a sequence of frames. The speech signal is encoded into a plurality of excitation pulses for each frame by the use of a pulse search method known in the art. Each of the excitation pulses has an amplitude and a location determined by the speech signal. Excitation pulse information is transmitted from the encoder to the decoder through a transmission medium. The decoder decodes the excitation pulse information into a decoded signal and outputs the decoded signal as a synthetic speech signal.
The number of the excitation pulses per single frame is determined by performance of the encoder. When the excitation pulses are reduced in number, the synthetic speech signal degrades in quality. On the other hand, the number of the excitation pulses determines a transmission rate of the data transmission system. Namely, the transmission rate reduces in proportion to the number of the excitation pulses. It is preferable to reduce the transmission rate in view of reduction of a transmission bandwidth. A recent demand is directed to a low transmission rate of, for example, 4.8 kbits/sec. In consideration of the number of the excitation pulses, the transmission rate of the encoder of the Tanaka et al patent must be 8 kbits/sec at the lowest.
In the meanwhile, pre-emphasis operation is well known in the art as a useful method which is capable of raising an SNR (signal to noise ratio) of the encoder. For this purpose, the encoder comprises a pre-emphasis filter which is for emphasizing high frequency component of the speech signal. The pre-emphasis filter flattens a spectrum envelope of the speech signal. In the case, the decoder needs a de-emphasis filter which has an inverse filter characteristic relative to the pre-emphasis filter.